The present invention relates to a degassing apparatus for a mold for breathing a mold cavity upon injection molding performed by an injection molding apparatus such as a die-casting machine or an injection molding press.
A conventional relevant technique similar to the present invention is described in Japanese Patent Laid-Open No. 63-60059. In this technique, a molten metal sensor constituted by two electrodes insulated from each other is short-circuited when a molten metal from a cavity reaches the sensor, and a switching circuit is immediately activated accordingly to energize an air solenoid valve or an electromagnetic coil, thereby closing a degassing valve.
In the above conventional technique, when a molten metal moves in the form of a mass from the cavity through a degassing passage, this molten metal can be easily detected. Therefore, since the degassing valve is closed, the molten metal is prevented from entering into the degassing valve. In actual casting, however, a molten metal from the cavity rarely moves in the form of a mass from the cavity through the degassing passage. Generally, the molten metal is often flaky or granular. Since the molten metal sensor is constituted by the two insulated electrodes, it cannot detect these flakes or grains. Therefore, the degassing valve is not closed, and the molten metal flakes or grains enter into the degassing valve and cut into the sheet surface of the valve. As a result, a large amount of the molten metal sometimes enters into the degassing valve.